1. Field of the Invention
The present invention relates to an optical fiber preform manufacturing method for manufacturing an optical fiber having holes extending in a longitudinal direction.
2. Description of the Related Art
Optical fibers having holes extending in a longitudinal direction, so-called microstructure optical fibers, have characteristics that cannot be achieved by optical fibers having a normal structure of confining light with a core and a cladding, and thus are expected to be optical fibers of the next generation. A holey fiber, a hole-assisted fiber, and a photonic bandgap fiber are known as the microstructure optical fibers.
The holey fiber has holes formed around a region that has a substantially uniform refractive index and that is near a center axis, and guides light with a core, which is the region near the center axis in which the holes are not formed. The hole-assisted fiber has holes formed around a core of an optical fiber of a normal structure. The photonic bandgap fiber has a hole formed on a center axis and becoming a core, and has holes cyclically formed around this hole, thereby generating a photonic bandgap to guide light.
A stack-and-draw method and a drilling method are known as methods of manufacturing a preform for a microstructure optical fiber. The stack-and-draw method is a method of manufacturing a preform for a microstructure optical fiber by bundling plural capillary tubes. The hole drilling method is a method of manufacturing a preform for a microstructure optical fiber by forming through-holes in a longitudinal direction of the preform with a drill (see Japanese Patent Application Laid-open No. 2002-321935 and Japanese Patent Application Laid-open No. 2003-342032).
The stack-and-draw method is suitable for arranging many holes in a constant cycle. However, in the stack-and-draw method, capillary tubes need to be prepared as many as the number of holes to be formed, and therefore procurement of parts is difficult. Furthermore, because unintended vacant space is formed between the capillary tubes, characteristics of the optical fiber can become unstable.
The drilling method is suitable for arranging a small number of holes at arbitrary positions in a cross-sectional direction. When a microstructure optical fiber is manufactured, desired optical-fiber characteristics need to be achieved by controlling arrangement positions and shapes of its holes precisely. Therefore, in manufacturing a preform for a microstructure optical fiber, the hole drilling method is used in many cases, particularly when the number of holes to be arranged is small.
However, when the drilling method is used, a hole having a length greater than that of a drill used in the drilling method cannot be formed in one drilling process. Specifically, the length of a drill is about 500 millimeters at most, and thus a hole having a length larger than 500 millimeters cannot be formed in one hole drilling process. Therefore, when the hole drilling method is used, there is a constraint on the length of manufacturable preforms. Accordingly, when the hole drilling method is used, manufacturing large preforms is difficult, and it is difficult to reduce the manufacturing cost of an optical fiber having holes extending in a longitudinal direction.